1. Field of the Invention
The present invention relates to a drive control circuit for a motor. More specifically, the present invention pertains to a drive control circuit that protects contacts of switches for controlling rotational directions of the motor.
2. Description of the Related Art
A conventional direct current motor of this type is employed for driving wipers mounted on a vehicle. As shown in FIG. 9, the conventional direct current motor 51 for driving wipers is electronically connected between a power supply 57 and a ground 58. The motor 51 has two connection terminals, a first terminal and a second terminal. The first terminal is connected with a first relay Ry52, and the second terminal with a second relay Ry53, respectively. A contact 55 of the first relay Ry52 is connected with the power supply 57, and a contact 56 thereof is connected with the ground 58. A movable contact 54 of the first relay Ry52 is selectively connected with either one of the contacts 55 or 56, by means of a switching control circuit (not shown).
A contact 60 of the second relay Ry53 is connected with the power supply 57, and a contact 61 thereof is connected with the ground 58. A movable contact 59 of the second relay Ry53 is selectively connected with either one of the contacts 60 or 61, by means of the switching control circuit (not shown).
Therefore, when the movable contact 54 of the first relay Ry52 is connected with the contact 55, and the movable contact 59 of the second relay Ry53 is connected with the contact 61, by means of the switching control circuit, the motor 51 rotates in the forward direction in response to current I10 flowing from the power supply 57 to the motor 51. On the contrary, when the movable contact 54 of the first relay Ry52 is connected with the contact 56, and the movable contact 59 of the second relay Ry53 is connected with the contact 60, by means of the switching control circuit, the motor 51 rotates in the reverse direction, in response to current I11 flowing from the supply 57 to the motor 51.
On the other hand, in order to cause the motor 51 to be stopped, the movable contacts 54 and 59 of the first and second relays Ry52 and Ry53 are respectively connected with the contacts 56 and 61, and each first and second connection terminals of the motor 51 is connected with the ground 58. Consequently, a closed loop including the motor 51 is formed. A current flow is generated in the motor in the opposing direction to the direction of the current flow during the motor 51 is regularly rotated. The current flow is caused by electromotive force generated by means of the force of inertia of the motor 51. Therefore, the motor 51 is immediately stopped.
However, when the movable contact 54 of the relay Ry52 is disconnected from the contact 55, and then connected with the contact 56, in order to stop the motor 51, electric discharge may occurs between the movable contact 54 and the contact 55, because of the potential difference between the contacts 55 and 56, which may cause the contacts 55 and 56 to electrically connect each other. In such a case, short-circuit current I12 flows to a motor drive circuit, from the power supply 57 via the contacts 55 and 56, a problem arises in that each of the contacts 55 and 56 is melted down and the first relay is damaged.